1. Field of the Invention
The present invention relates to a coherent light source, and more particularly to a coherent light source being usable in a wavelength division multiplexing system.
2. Description of the Related Art
In a passive wavelength-division multiplexing system, a signal is transmitted to a large number of subscribers by means of a single optical fiber. The commonest type of the passive wavelength-division multiplexing system is a double star structure, in which a local base station, connected to a central base station by a single optical fiber, is located adjacent to a plurality of the subscribers, and is connected to each of the subscribers.
U.S. Pat. Ser. No. 6,411,410, filed in 2002 by Ian Robert Wright, et al., and entitled “WAVELENGTH-DIVISION MULTIPLEXING IN PASSIVE OPTICAL NETWORK”, discloses the above-described passive wavelength-division multiplexing system. Downstream channels to subscribers are multiplexed to an optical signal which is transmitted to the local base station. The local base station demultiplexes the received signal among downstream channels having different wavelengths, and transmits the downward channels to each of the subscribers.
A semiconductor optical amplifier such as a Fabry-Perot laser is a semiconductor optical element that amplifies inputted light, and is used as a light source for generating the above-described downstream channels. The above passive wavelength-division multiplexing system injects light having a predetermined wavelength into the laser, thereby generating a mode-locked downstream channel having the same wavelength a s that of injected light.
FIG. 1 is a cross-sectional view of a conventional Fabry-Perot laser. With reference to FIG. 1, the above conventional Fabry-Perot laser has a MESA structure comprising a semiconductor substrate 110, a lower cladding layer 120 grown on the semiconductor substrate 110, an active layer 130 formed on the lower cladding layer such that a band gap in the active layer is uniformly distributed, an upper cladding layer 140 grown on the active layer, and a contact layer 150 stacked on the upper cladding layer.
Since the active layer 130 of the Fabry-Perot laser has a uniform band gap distribution, the Fabry-Perot laser generates a downstream channel in which a mode of a single wavelength is locked.
However, the band gap of the active layer is varied not only intentionally by a driving current, but easily varied unintentionally due to variation with external temperature. This latter effect on the band gap may cause a gain wavelength of the Fabry-Perot laser to stray from the wavelength of light inputted into the Fabry-Perot laser. The resulting mismatch reduces the gain of the downstream channel generated by the Fabry-Perot laser.